This invention relates to a method of beneficiating titania slag to a high grade titanium dioxide (TiO2) product. Preferably the product is suitable for use as a feedstock in titanium dioxide pigment production or titanium metal production by means of the chloride process. The invention also relates to a process for preparing intermediate products suitable for use in the beneficiation of titania slag and also to intermediate products and final products formed by the processes.
More particularly the process of the present invention includes the steps of sizing the slag; oxidizing the sized slag and then reducing the oxidized slag. The treated slag may then be subjected to steps such as acid leaching.
Commercial Uses of TiO2 
Titanium is widely known for its use as a metal, but the primary use of titanium is in the form of titanium dioxide (TiO2). TiO2 is used as a white pigment in paints, plastics and paper. Two types of pigment with a tetragonal crystal structure are produced, namely rutile and anatase. Rutile is preferred in outdoor paints and anatase is preferred in indoor paints.
TiO2 Pigment Production
There are two commercial processes for the production of TiO2 pigment namely, the sulphate process and the chloride process. A sulphate process plant is easier to operate and monitor than a chloride process plant, and is capable of using feedstock with a relatively low TiO2 content. However, capital costs of a modem sulphate process plant can be higher than that of a chloride process plant of the same pigment capacity. Furthermore there is a higher volume of waste products to be treated and disposed of due to the use of more impure feedstock and the fact that the sulphate used in the process cannot be easily recovered and recycled.
Accordingly the chloride process is a more popular process and is growing in popularity. The feedstock suitable for use in the chloride process usually needs to have a high TiO2 content and needs to contain fewer impurities than those suitable for the sulphate process.
TiO2 Bearing Deposits
TiO2 is commonly found in nature in the form of ilmenite (FeO. TiO2) which contains from 40% to 80% TiO2. Most deposits being mined produce concentrates with a TiO2 content between 45% and 67%. Rutile deposits are far more scarce than ilmenite and they contain about 95% TiO2 in crystalline form and are therefore of sufficient quality to be used directly in the chloride process for TiO2 pigment production. Deposits of anatase have been discovered but have not yet been commercially exploited. Anatase typically has a TiO2 content in excess of 95%. Leucoxene, a weathered form of ilmenite, contains up to 85% TiO2 and is exploited on a limited commercial scale. Brookite (rhombic TiO2), perovskite, (CaTiO2), sphene (CaTiSiO5) and geikielite (MgTiO3) also contain titanium.
Beneficiation of Ilmenite
Although natural rutile is suitable for use as a feedstock in the chloride process, the ever. decreasing availability of natural rutile forced chloride process pigment producers to consider other lower grade feedstock. One such alternative is naturally occurring ilmenite. Due to its relatively low TiO2 content several processes have as their aim the upgrading of the TiO2 content of ilmenite.
These processes include:
i) Partial Reduction of the Iron in the Ilmenite.
This process is described in U.S. Pat. Nos. 4,038,364 and 4,199,552. In this process ilmenite is reduced at elevated temperatures to convert iron in the ferric state, (Fe(III)), to the ferrous state, (Fe(II)). This renders the iron more amenable to acid leaching of the ilmenite during upgrading of the ilmenite.
ii) Pre-Oxidation Followed by Partial Reduction of the Iron in the Ilmenite.
In a process described in GB1,225,826 the ilmenite is subjected to an oxidation treatment to convert substantially all the iron to the ferric state. The ore is then reduced to convert the iron back to the ferrous state and metallic state. In the examples of the patent the oxidation is carried out at 870xc2x0 C. for two hours. The reduction is carried out at 870xc2x0 C. for five minutes. The ore exhibits the original X-ray diffraction pattern of ilmenite after treatment but is more amenable to acid leaching to upgrade the ilmenite.
iii) Pre-Oxidation Followed by Reduction of the Iron to Metallic State.
U.S. Pat. No. 4,097,574 describes a process whereby ilmenite is subjected to an oxidation treatment to convert the iron in the ilmenite to the ferric state. Reduction treatment is then carried out to reduce the iron to metallic iron. The iron is then removed by leaching thereby to upgrade the ilmenite.
iv) Smelting of the Ore.
Ilmenite ore can also be smelted in the presence of a carbonaceous reducing agent in an electric arc furnace. This process is described in U.S. Pat. No. 2,680,681. Two saleable products result from this namely, high quality pig iron and titania rich slag. The slag typically contains 80-85% TiO2.
Differences Between Ilmenite Ore and Titania Slag
All of the processes listed above are aimed at beneficiating ilmenite or similar titanium ores. None of these processes were applied to titania slag and there are certain fundamental differences between ilmenite ore and titania slag.
i) The first difference is that ilmenite is a naturally occurring titanium bearing ore, while titania slag is produced by electro-smelting of ilmenite in an electric arc furnace.
ii) The second difference can be found in the amount of the main components that are present. Ilmenite typically contains around 50% titanium oxide and around 45% iron oxide. All the titanium is present as Ti(IV) while around 20% of the iron occurs as Fe(III) and the rest is in the Fe(II) state. Titania slag typically contains around 85% titanium oxide and around 10% iron oxide. In this instance the titanium is in the Ti(III) and the Ti(IV) state, while most of the iron is present as Fe(II).
iii) The third difference lies in the respective mineralogical compositions. In ilmenite concentrates the iron and the titanium is organised into hexagonal ilmenite crystals. As-cast titania slag consists of the following four phases:
a) The most abundant phase is a crystalline phase, known as pseudobrookite or the M3O5 phase. This phase is a solid solution of iron oxide and titanium oxide, with the end members being (Ti,Fe,Al,Cr,V)2O3.TiO2 and (Mg,Mn,Fe)O.2TiO2 and can accommodate the main oxidation states of iron and titanium in its structure, namely Fe(II), Fe(III), Ti(III) and Ti(IV);
b) Rutile (TiO2) although not always present in such quantities that allows detection thereof by X-ray diffraction analysis;
c) An amorphous, glassy phase consisting mainly of SiO2, TiO2, FeO, CaO and Al2O3 and;
d) Finely disseminated metallic iron globules present in the grain boundaries of the rutile crystals and in the silicate-rich glassy matrix.
The pseudobrookite and amorphous glassy phases are characteristic of titania slag and generally do not occur in ilmenite ores. The presence of pseudobrookite and the glassy phases in titania slag may be one of the causes that the processes for beneficiating ilmenite ore are in some cases not applicable to the beneficiation of titania slag. The different compositions of slags may also play a role.
Beneficiation of Titania Slag
Several known processes have as their aim the upgrading of the TiO2 content of titania slag.
These processes can be classified as follows:
i) Chlorination of the Impurities
A process described in U.S. Pat. Nos. 4,629,607; 4,933,153; 5,063,032 and 5,389,355 to upgrade titania slag containing at least one alkaline earth impurity. Firstly the slag is preheated in a fluidized bed reactor in an atmosphere void of oxygen to prevent the oxidation of the Ti(III) present in the slag to Ti(IV). The slag is then contacted with hydrogen chloride gas. This results in the formation of iron and alkaline earth chlorides in the slag. Finally the chlorides that formed during the chlorination treatment are leached with either water or hydrochloric acid.
ii) Salt Roasting
In U.S. Pat. No. 4,038,363 a process for the upgrading of slag is described. The process consists of a roast procedure in the presence of an alkaline salt such as sodium chloride. After the roast procedure the agglomerates that have formed are dispersed with wet grinding. Thereafter the slag is subjected to leaching in either water or a sulphuric acid solution.
iii) Fluxing of the Impurities
Titania slag can also be upgraded by heating the slag in the presence of a glass forming fluxing agent such as phosphorus pentoxide as is described in U.S. Pat. No. 3,996,332. According to South African patent 93/5922 other glass forming agents such as the oxides of sodium, potassium, silicon etc. can also be used. After the fluxing procedure the slag consists of a crystalline rutile phase and a glassy phase that contains most of the impurities present in the slag. Finally the slag is subjected to leaching in a mineral acid to remove the glass phase and associated impurities.
iv) Sulphatising
U.S. Pat. No. 4,362,557 describes a process where the TiO2 content of titania slag is increased in a two stage procedure. Firstly the slag is mixed with an alkaline salt such as sodium carbonate and reacted with either SO3 or mixtures of SO2 and O2 at 700 to 1100xc2x0 C. Secondly the sulphates that formed during the roasting are leached with either water or hydrochloric acid at room temperature.
v) Oxidation-Reduction Roasting
The process described in patent application PCT/CA96/00767 has as its basis an oxidation roast followed by a reduction roast. The slag is first sized in the range 75-850 xcexcm and is then oxidised at a temperature of at least about 950xc2x0 C., but preferably between 1000 and 100xc2x0 C., for at least 20 minutes. The oxidation procedure converts the Fe(II) and Ti(III) present in the slag to Fe(III) and Ti(IV) respectively and aims to decompose the glassy phase. After the oxidation the slag is reduced at a temperature of at least about 700xc2x0 C., but preferably between 800 and 850xc2x0 C., for at least 30 minutes, but preferably for a period of 1,5 to 2 hours, to convert the Fe(III) in the slag back to Fe(II). A MgO rich ilmenite-geikielite solid solution forms during the process, which is more amenable to leaching than the original phases present in the slag. The roasted titania slag is then leached under pressure in excess of atmospheric pressure and at a temperature of at least 125xc2x0 C. to remove the impurities present in the slag.
Patent application PCT/CA96/00767 referred to in the above paragraph also stresses the differences between the treatment of ilmenite and titania slag. In example 12 the application illustrates that the process of GB 1,225,826 relating to the treatment of ilmenite (as discussed Add, above) is not suitable when applied to titania slag. As in the case of the process of PCT/CA96/00767, the process of GB1,225,826 includes an oxidation and subsequent reduction treatment. However, negligible removal of impurities are achieved when the process of GB1,225,826 is applied to slag, that is by oxidizing the slag with air at 850xc2x0 C. for 2 hours and then reducing it with smelter gas at 850xc2x0 C. for 5 minutes and thereafter leaching the resulting product with a hydrochloric acid solution under reflux conditions. Even if the process is modified by carrying out the oxidation at 900xc2x0 C. for 1 hour and the reduction at 900xc2x0 C. for 30 minutes (as set out in example 13 of PCT/CA96/00767) very poor results are achieved.
Patent application PCT/CA96/00767 teaches that the titania slag requires a pre-treatment within an unexpected window of process conditions to render it suitable for acid leaching. The patent describes much harsher oxidation, reduction and acid leaching steps for slag than the conditions for ilmenite as disclosed in the related process of GB 1,225,826.
Most surprising it has now been found that if titania slag is oxidized at a lower temperature than that described in patent application PCT/CA96/00767 under the correct conditions and thereafter reduced and further treated, the slag can be suitably upgraded. In some embodiments of the invention it is not necessary to carry out the leaching at above atmospheric pressure. Leaching at a pressure above atmospheric pressure is required in the process of PCT/CA96/00767. It will be appreciated that even if leaching at above atmospheric pressure may not be necessary for the successful beneficiation of titania slag according to the invention, the process will also function if acid leaching is carried out at above atmospheric pressure.
Patent application PCT/CA96/00767 teaches that during that process the iron cations tend to concentrate around pores formed in the slag particles which will render them more accessible to leaching. It is believed that if the oxidation step is carried out at lower temperatures as disclosed for the present invention the iron in the slag particles surprisingly migrates to the rims of the slag particles. It is believed that such slag particles undergo rapid reduction roasting and that such slag particles are more amenable to acid leaching which allows leaching to be conducted at atmospheric pressure.
According to the present invention a method of treating titania slag to increase the leachability of the slag comprises the steps of
sizing the titania slag to a particle size from 75 to 850 xcexcm;
oxidizing the sized slag particles in an oxidizing atmosphere at a temperature from about 700xc2x0 C. and above but below about 950xc2x0 C. for at least 30 minutes allowing the iron present in the slag to concentrate at the exposed surfaces of the slag particles, allowing a major portion of the iron in the Fe(II) state to convert to the Fe(III) state, and allowing the titanium in the Ti(III) state to be converted to the Ti(IV) state; and
reducing the oxidized slag in a reducing atmoshpere from about 700xc2x0 C. to about 950xc2x0 C. for at least 5 minutes to convert a major portion of the iron in the Fe(III) state to the a FE(II) state without converting a substantial amount of the titanium in the Ti(IV) state to the Ti(III) state.
According to another aspect of the present invention a method of treating titania slag to increase the leachability of the slag comprises the steps of
sizing the titania slag to a particle size from 75 to 850 xcexcm;
oxidizing the sized slag particles in an oxidizing atmosphere at a temperature from about 700xc2x0 C. and above but below about 950xc2x0 C. for at least 30 minutes allowing an anatase phase to stabilize in the slag, allowing a major portion of the iron in the Fe(II) state to convert to the Fe(III) state, and allowing the titanium in the Ti(III) state to be converted to the Ti(IV) stage; and
reducing the oxidized slag in a reducing atmosphere from about 700xc2x0 C. to about 950xc2x0 C. for at least 5 minutes, to convert a major portion of the iron in the Fe(III) state to the Fe(II) state and without converting a substantial amount of the titanium in the Ti(IV) state to the Ti(III) stage.
The titania slag includes a pseudobrookite phase and a glassy phase. The glassy phase may consist mainly of SiO2, TiO2, FeO and Al2O3.
The titania slag may contain titanium oxide and impurities including at least one compound selected from the group consisting of iron oxide, silicon oxide, aluminium oxide, alkaline earth oxide, manganese oxide, chromium oxide and vanadium oxide. The titanium oxide and impurities may be provided in a pseudobrookite phase and a glassy phase. The alkaline earth oxide may comprise calcium oxide and/or magnesium oxide.
The titania slag is preferably crushed and preferably to a particle range of above 106 xcexcm up to 850 xcexcm.
During the oxidation step the iron present in the slag preferably concentrates at the exposed surfaces of the slag, and an anatase phase is allowed to stabilize in the slag.
The oxidation is preferably carried out at a temperature from about 750xc2x0 C. and above but preferably below about 900xc2x0 C. and more preferably is carried out at a temperature from about 800xc2x0 C. to about 875xc2x0 C.
The oxidation is carried out for longer than 30 minutes. Preferably it is carried out for a period of about 2 hours.
The oxidation is preferably carried out in a fluidized bed reactor.
The oxidizing atmosphere may comprise oxygen diluted by an inert gas (preferably a mixture of Co2 and N2) containing at least 2% oxygen by volume. More preferably the atmosphere results from the combustion of a carbonaceous fuel with excess air. Most preferably the oxidizing atmosphere contains between 4% and 8% oxygen by volume.
Preferably more than 60%, preferably more than 75%, more preferably more than 90% and most preferably substantially all the iron in the Fe(II) state is converted to the Fe(III) state during oxidizing of the slag. The reduction is preferably carried out at a temperature between about 800xc2x0 C. and about 875xc2x0 C.
The reduction is preferably carried out in a fluidized bed reactor.
The reducing atmosphere may be supplied by any one of the following reducing agents: carbon monoxide gas, hydrogen gas, gases such as reformed natural gas and smelter off gas and mixtures between these gases. More preferably the reducing atmosphere is supplied by the products resulting from combustion of coal.
The reduction is preferably carried out for a period of longer than 10 minutes and less than 1 hour. More preferably it is carried out for a period of 20 minutes.
Preferably more than 60%, preferably more than 75%, more preferably more than 90% and most preferably substantially all iron in the Fe(III) state is converted to the Fe(II) state during reduction.
Most preferably, none of the titanium in the Ti(IV) state is converted to the TI(III) state during reductions.
According to another aspect of the invention there is provided a method of beneficiating titania slag to increase the TiO2 content thereof comprising the steps of:
sizing the titania slag to a particle size from 75 to 850 xcexcm;
oxidizing the sized slag particles in an oxidizing atmosphere at a temperature from about 700xc2x0 C. and above but below about 950xc2x0 for at least 30 minutes allowing the iron present in the slag to concentrate at the exposed surfaces of the slag particles, allowing a major portion of the iron in the Fe(II) state to convert to the Fe(III) state, and allowing the titanium in the Ti(III) state to be converted to the Ti(IV) state;
reducing the oxidized slag in a reducing atmosphere from about 700xc2x0 C. to about 950xc2x0 C. for at least 5 minutes to convert a major portion of the iron in the Fe(III) state to the Fe(II) state and without converting a substantial amount of the titanium in the Ti(IV) state to the Ti(III) state; and
leaching the reduced slag with acid to obtain a beneficiated slag product with an increased TiO2 content and leach liquor containing the leached impurities.
According to another aspect of the invention there is provided a method of beneficiating titania slag to increase the TiO2 content thereof comprising the steps of:
sizing the titania slag to a particle size from 75 to 850 xcexcm;
oxidizing the sized slag particles oxidizing atmosphere at a temperature from about 700xc2x0 C. and above but below about 950xc2x0 C. for at least 30 minutes, allowing an anatase phase to stabilize in the slag, allowing a major portion of the iron in the Fe(II) state to convert to the Fe(III) state, and allowing the titanium in the Ti(III) state to be converted to the Ti(IV) state;
reducing the oxidized slag in a reducing atmosphere from about 700xc2x0 C. to about 950xc2x0 C. for at least 5 minutes to convert a major portion of the iron in the Fe(III) state to the Fe(II) state and without converting a substantial amount of the titanium in the Ti(TV) state to the Ti(III) state; and
leaching the reduced slag with acid to obtain a beneficiated slag product with an increased TiO2 content and leach liquor containing the leached impurities.
The leaching may be conducted under pressure in excess of atmospheric pressure. Alternatively the leaching may be conducted at atmospheric pressure. Alternatively, a combination of atmospheric and pressure leaching may be used.
The acid may be heated and preferably the acid is heated to the boiling point of the acid.
The acid may comprise a mineral acid and preferably it comprises sulphuric acid or hydrochloric acid, more preferably hydrochloric acid.
The acid may be present in at least a 10% stoichiometric excess of what is needed to convert leachable oxides and alkaline impurities to soluble chlorides.
The acid leaching may be done in one or more stages. If more than one stage is used then the leaching may be done in co-current or counter-current mode.
The leaching may be done in batch or continuous mode.
The method of beneficiating titania slag may optionally include a caustic leaching step after the acid leaching step.
Optionally the method includes calcination of the treated slag. Prior to calcining the treated slag it may be washed and it may be dried to remove volatile by-products. The drying step may be carried out at a temperature above 100xc2x0 C.
The calcination may be carried out by heating the product between 600xc2x0 C. and 900xc2x0 C. for more than 30 minutes.
The method may also include an additional step of subjecting the calcined slag to a magnetic separation procedure.
The methods of beneficiating the titania slag is preferably performed to form beneficiated titania slag suitable for use as a feedstock for the chloride process of TiO2 pigment production.
The beneficiated titania slag may contain at least 90% by weight, preferably at least 94% by weight of titanium dioxide. Preferably it contains less than 1,5%, preferably less than 1% by weight of magnesium oxide. Preferably it contains less than 0,4% by weight of calcium oxide.
The invention also relates to products formed by the methods described herein above.
According to another aspect of the invention there is provided treated titania slag containing rutile, anatase and pseudobrookite.
According to yet another aspect of the present invention there is provided treated titania slag including rutile, anatase, ilmenite and pseudobrookite.